1. Field of the Invention
The present invention relates to an apparatus for processing solid refuse and, in particular, to a device for shredding solid refuse that employs a shaft equipped with radial blades.
2. Description of the related Art
Ecological problems have become a primary concern in modern society. One of the major problems is the disposal of solid waste which, of any kind, can create problems with the soil and water stratums as well as air pollution when the solid waste is incinerated. Indeed, solid waste management is of critical importance in view of the incredible amount of solid waste generated daily, particularly in large metropolitan areas. However, safe, reliable, and effective methods of dealing with solid waste have yet to be developed.
To date, the trend have been to dump solid waste in landfills. However, in addition to the environmental and health concerns of landfills, landfill capacity is a primary problem as available landfills are rapidly being exhausted as a result of the high volume of solid waste generated. Because the capacity of landfills is being expended so rapidly, several efforts have been made to reduce waste volume before dumping.
For example, there has been a great deal of interest in municipal and commercial solid waste processing through recycling, in marketable material recovery, as well as in the preparation of RDF (Refuse Derived Fuel) for energy production. However, in order to implement the above waste reduction practices, it is necessary to prepare the waste by clearing plastic and paper bags used to wrap the waste and by reducing the volume of oversized items such as pieces of cardboard and wood which can generate clogs in a processing line.
The removal of wrappings and reduction in size of oversized items has been attempted with varying kinds of crushing equipment. In fact, many different kinds of crushing equipment have been used and manufactured worldwide in order to perform waste volume reduction and/or bag breaking. Unfortunately, to date these devices have only been of limited usefulness and thus have not met their original expectations.
Traditional high speed hammer mills have been frequently used and while they can solve the problem of breaking bags and reducing volume, they have created a series of other problems. For example, there is a danger of explosion due to formation of a milieu stowed with paper fibers and inflammables with the contemporaneous generation of sparks by the friction of metallic parts. In addition, there are high operation and maintenance costs due to the high energy consumption and considerable abrasion of the internal parts of the system. Further, when the equipment is used upstream of a recycling process, the mutual contamination of the materials which define the waste flow is an additional problem.
Some of the equipment for processing solid refuse have been known for a number of years. However, in spite of attempts to overcome the above disadvantages including the application of low speed equipment, in most cases the expected results have not been achieved especially in terms of hourly throughput and problems created by unshreddable items present in the waste flow. One system is shown in U.S. Pat. No. 4,039,150. A second system is shown in Japanese Patent No. 5183265 and a third system is shown in U.S. Pat. No. 4,489,896. All of these systems are based on the same principle which is represented by a rotating drum equipped with projecting blades which penetrate into a series of fixed, comb-like counterblades with different shapes.
Specifically, the main feature of the system described in U.S. Pat. No. '150 is represented by an input material feeding system and a compressing system towards the shredding stage. Therefore, this device is particularly suited for shredding bulky items. However, the shredding stage has considerable limits. More particularly, while the machine can reject hard, unshreddable items, it can not separate such items from the remaining mass flow. In addition, while the rotating drum contrast anvil operates by virtue of its own weight and inertial force, the drum carrying the radial blades can not be low speed since it requires a considerably high impact velocity between the drum blades and the anvil so as to prevent the anvil from opening and staying open. Thus, the required impact velocity can only be reached with a medium high speed.
A further problem with the system described in U.S. Pat. No. '150 is that all the materials to be processed pass through the blades and counterblades. Therefore, abrasive components present in the material flow such as glass, grit, stones, etc. can quickly wear the radial blades and the anvil. An even further problem is with the infeed system. Indeed, this system can cause considerable problems when the materials to be fed are municipal solid waste since oversized rags, ropes, films, pantyhose, etc. tend to twist around the rotating device and cause the same to stop rotating after a relatively short time.
The Japanese patent shows an apparatus for processing a particular kind of waste, not municipal solid waste. This device is adapted to shred the material between teeth mounted on a rotating drum and a series of shredding blades which are kept in a desired position by a pneumatic spring. The shredded material is passed on a grate which accepts shredded material while it carries unshreddable items forward to a special outlet by the thrust of the rotating teeth. As is apparent, then, this system can only work with very peculiar waste because in the case of municipal solid waste which consists of mixed materials of many kinds such as plastic, rags, paper, etc. the ground material would not pass through any grate but would be dragged towards the rejection outlet together with the unshreddable items. The problems of this dragging effect are readily apparent. Even if this machine may possibly, with special modifications, process some kinds of municipal solid waste, it would present, in any case, several problems. Specifically, the rotating drum can not be a low speed drum since a centrifugal force is required to allow the shredded material to pass through the grate and the rejected material to pass through the discharge outlet. Further, the entire waste flow passes through the drum blade shredding blade system. Thus the entire abrasive material fraction is passed therebetween and can cause considerable wear on these members and the grate below.
U.S. Pat. No. '896 provides a machine suited for any kind of waste with a slow rotating operation and also with the possibility of discharging unshredded waste. However, this system has limits as well. In particular, it has a very restricted hourly capacity due to unshreddable item expulsion which is performed by rotor catch and a reversing of the rotor. A great amount of time lost because this cycle catch, reversing and resetting. Second, the rotor speed must be very low so that the device can stop when jammed without causing damage to the mechanical members. Finally, all the material to be processed must pass through the shredding teeth and thus abrasive material which is present in the flow can quickly wear the contact elements.